Projection welding is a known technique for permitting a weld-type securement between two thin overlapping metal sheets. One of the sheets is provided with a small projection extending transversely therefrom with the tip of the projection being maintained in contact with the other overlapping sheet. The electrode of a welding gun is normally pressed into contact with the other sheet, generally in alignment with the projection. A combination of force and electric current is then applied to the electrode to effect welding together of the two sheets at the contact area defined by the projection, and to collapse the projection to effect a weld nugget for securing the overlapping sheets together. Conventional projection welding technique applies alternating electrical current to the contact electrode over a relatively long time period, whereby several cycles of AC current are applied to the electrode to permit performance of a single projection weld. The long current application time and the difficulty in achieving the desired contact force and collapsing of the heated projection have often resulted in disadvantages which have made projection welding less desired for use on thin gauge metal sheets. For these and other reasons, projection welding has previously not met with a high level of acceptance for use on thin metal sheets, although more recently a greater acceptance is being achieved.
To further explain conventional projection welding systems, there is illustrated in FIG. 8 a known system 10 for effecting projection welding. This welding system 10 is particularly desirable for projection welding of thin sheet-like metal components disposed in overlapping relation, typically light-gauge metal sheets which commonly have a thickness in the range of about 0.020 to about 0.050 inch. The thin sheets as illustrated at 11 and 12 have portions which directly overlap, and in a hem welding process one of the sheets 11 has a flange or hem part 13 which is bent upwardly to overlap a side of the other sheet 12. The sheet 12 is sandwiched between the sheet 11 and its hem part 13. Intermediate sheet 12 has a bead or projection 14 projecting transversely therefrom so that the tip of the projection contacts the adjacent surface of the hem part 13. A projection weld is created directly between the sheet 12 and the hem part 13 at the region of the projection 14.
In the projection welding system 10 as shown in FIG. 8, the overlapping sheets 11, 12 are typically positioned on a support or die 15, and a movable welding held assembly 17 is positioned adjacent the die 15 to effect creation of the projection weld between the overlapped sheets. The welding head assembly 17 includes a hollow housing or body 18 having an electrode 19 movably supported therein and projecting outwardly for contact with the overlapped sheets in the region of the projection, and a spring 21 is confined within the housing 18 and acts against an inner face of the electrode 19 so as to urge the electrode outwardly into an extended position, in which position the electrode abuts an interior stop surface formed on the housing. The welding head assembly 17 is electrically connected to a stationary transformer 22. The power supply to the transformer 22, and hence the welding current supplied to the welding head assembly, is controlled by a suitable control unit 23. The transformer 22 has the primary coils 24 thereof connected to suitable electrical conductors 25 and 26 that supply electrical energy to the welding system 10. The secondary coils 27 of the transformer are in turn connected to electrical conductors 28 and 29, one of which is connected to the welding head assembly 17 and the other is connected to the workpieces 11, 12, such as being connected either to the die 15 or to an electrical contact gun 31. The contact gun 31 includes a support 32 such as a conventional double-acting pressure cylinder, normally an air cylinder, having a conventional electrode 33 movably supported thereon. The electrode 33 engages a part 34 of the intermediate sheet 12, which part 34 is spaced from the overlapping portions of the sheets. The conductors 28 and 29 are typically constructed of a conventional flexible lamination so as to permit respective movement of the welding head assembly 17 and contact assembly 31 relative to transformer 22.
The welding head arrangement also includes a drive device 36 for effecting movement of the welding head assembly 17. The drive device conventionally comprises a pneumatic cylinder 37 having a housing 38, which is typically stationarily mounted spaced and separate from the welding head assembly 17. An extendible and contractible piston rod 39 extends from the housing 38 and couples the housing 18 of the welding head assembly 17 to the drive device 36. The piston rod 56 controls the movement of the assembly 17 into engagement with the sheets 11, 12 when a projection welding operation is carried out.
U.S. Pat. Nos. 4,417,122 and 5,714,730, both owned by the Assignee hereof, relate to improved projection welding arrangements which overcome some of the prior difficulties in projection welding light-gauge metal sheets. The '122 and '730 patents are herein incorporated by reference. Brief descriptions of the '122 and '730 patents are provided for convenience.
The system of the '122 patent relates to a welding gun having a lower inertia and hence fast response electrode receiving an extremely short duration electrical current, which duration is typically only a fraction of one-half of the alternating current wave cycle. A high energy, short duration pulse of current is transmitted to the moving welding head and applied to the projection contact area between the overlapping sheets to effect rapid and efficient welding of the sheets together.
The arrangement of the '730 patent is related to precalibrating a welding gun in the welding arrangement so that an electrode of the welding gun applies a force having substantially a predetermined magnitude against one sheet when creating the projection weld. The '730 welding head assembly is slidably mounted to a fixed frame and is activated by a drive cylinder mounted separate from the head assembly. The drive cylinder has an outwardly projecting rod that connects to the welding head assembly whereby the welding head assembly, including the calibration spring structure, is spaced from the drive cylinder. The piston rod facilitates movement of the welding head assembly for performing the projection weld. The precalibration of the electrode is permitted by pretensioning the spring, as housed in the welding assembly, against which the electrode is positioned. The spring yields to the driving force of the drive cylinder once the electrode contacts a workpiece and is further compressed. Once the drive cylinder extends the rod a predetermined amount, the spring force is precalibrated by adjusting a spring support surface to yield the desired force for performing the projection weld.
While projection welding and particularly hem-type projection welding using the aforementioned welding heads has proven fairly successful and is gaining commercial acceptability, nevertheless there still exists room for improvement. For example, with this type of welding head, it is still desired to reduce the linear size and integrate components to reduce the size and weight of the welding assembly making it easier to integrate into automated systems. Further, it is desired to reduce the mass of the components that move during the weld thereby reducing the inertia of the device to correspondingly reduce the required driving force so that the welding head has an efficient and rapid response time.
It is an object of this invention to provide an improved projection welding arrangement, which is believed to represent an improvement over the prior arrangements.
More specifically, in the improved welding arrangement of this invention, the welding head has a drive cylinder for axially moving the electrode assembly to transversely contact the workpiece. A piston rod extends from the cylinder and has an axial bore therein receiving a force generation unit therein, which imparts motion to a coaxial shaft slidably mounted in the rod bore and extends therefrom. The extension of the shaft mounts an electrode assembly thereon. An adjustable end assembly is selectively movably mounted in an open end of the rod bore and has an opening through which the shaft extends. The position of the end assembly in the rod bore preloads the spring to select a spring force.
In the illustrated embodiment, the force generation unit includes a preloadable, axially acting spring that is preloaded by selectively compressing the spring between two support surfaces and adjusting one of the support surfaces by positioning the end assembly in the rod bore.
Other objects and purposes of the invention will be apparent to person familiar with structures and processes of this general type upon reading the following description and inspecting the accompanying drawings.